It has been the conventional practice to mount ceramic wall flow monolith particulate filters of the type disclosed, for example, in U.S. Pat. No. 4,276,071, entitled Ceramic Filters for Diesel Exhaust Particulates issued June 30, 1981 to Robert J. Outland, in a manner similar to that normally used for supporting ceramic monolith type catalytic converters or reactors in their respective associate reactor housings.
Thus it has been the usual practice to support a ceramic monolith type catalytic reactor cone element about its outer peripheral surface along its full axial extent whereby it is held immovable and isolated from the inner surfaces of its associate housing in a manner as disclosed, for example, in U.S. Pat. No. 4,335,078, entitled Catalytic Reactor for Automotive Exhaust Line issued June 15, 1982 to Ushijima et al. This type of support arrangement has previously been deemed necessary, since the core element being made of ceramic material was considered fragile and thus it was believed that it required such support along its full length.
The usual arrangement to thus support the core element is by the provision of a cushioning or buffer layer, in the form, for example, of a mat of a suitable refractory type material, being applied about the outer periphery of the core element. As assembled in an associate housing, this material is sandwiched, in a predetermined compressed condition, as desired, between the outer peripheral surface of the core element and the inner peripheral surface of the shell portion of the associate housing. This mat material is suitably compressed or is otherwise formed whereby to effect a seal between the core element and the internal wall of its housing to prevent the bypass flow of exhaust gas.
Thus to date it has been deemed necessary to support a ceramic wall flow monolith particulate filter in its housing in a similar manner by the use of a mat material sandwiched between its outer peripheral surface and the internal surface of the shell portion of its associate particulate trap housing along the full length of the monolith particulate filter.
It has now been discovered that the above-described continuous support arrangement, as applied to a wall flow ceramic monolith particulate filter, has caused two problems to exist, as follows:
(1) Since the ceramic monolith particulate filter will expand and contract at a different rate than the steel housing, then in view of the tightly sealed full axial support of this filter on its whole length within the housing, radial and axial stresses can result in the monolith particulate filter, as during incineration of the particulates collected thereon, which can easily exceed its modules of rupture; and,
(2) The tight packing of the buffer layer around the exterior of the monolith particulate filter tends to conduct heat to the housing exterior, so that during the heating and then incineration of the particulates, the temperature within the interior of the filter is increased rapidly whereas the outermost cells thereof do not respond rapidly and, accordingly, thermal gradients will occur which can cause a thermal stress crack in the monolith particulate filter. Of course, if the aspect ratio (length.div.diameter) of the ceramic monolith particulate filter is large (&gt;1.0), the mechanical stress created by axial and radial temperature gradients are substantially increased.